Loop backwards using indices in Python?

The Question :

315 people think this question is useful

I am trying to loop from 100 to 0. How do I do this in Python?

for i in range (100,0) doesn’t work.

The Question Comments :
  • BTW, you probably want to loop from 99 to 0, and rarely from 100 to 0. This affects the answers.
  • @Acumenus That was exactly what I searched for when reached here. And for the record the solution is to simply write: range(100)[::-1] (which is automatically translated to range(9, -1, -1)) – Tested in python 3.7

The Answer 1

435 people think this answer is useful

Try range(100,-1,-1), the 3rd argument being the increment to use (documented here).

(“range” options, start, stop, step are documented here)

The Answer 2

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In my opinion, this is the most readable:

for i in reversed(xrange(101)):
    print i,

The Answer 3

40 people think this answer is useful
for i in range(100, -1, -1)

and some slightly longer (and slower) solution:

for i in reversed(range(101))

for i in range(101)[::-1]

The Answer 4

18 people think this answer is useful

Generally in Python, you can use negative indices to start from the back:

numbers = [10, 20, 30, 40, 50]
for i in xrange(len(numbers)):
    print numbers[-i - 1]



The Answer 5

12 people think this answer is useful

Why your code didn’t work

You code for i in range (100, 0) is fine, except

the third parameter (step) is by default +1. So you have to specify 3rd parameter to range() as -1 to step backwards.

for i in range(100, -1, -1):

NOTE: This includes 100 & 0 in the output.

There are multiple ways.

Better Way

For pythonic way, check PEP 0322.

This is Python3 pythonic example to print from 100 to 0 (including 100 & 0).

for i in reversed(range(101)):

The Answer 6

6 people think this answer is useful

Another solution:

z = 10
for x in range (z):
   y = z-x
   print y



Tip: If you are using this method to count back indices in a list, you will want to -1 from the ‘y’ value, as your list indices will begin at 0.

The Answer 7

5 people think this answer is useful

The simple answer to solve your problem could be like this:

for i in range(100):
    k = 100 - i

The Answer 8

1 people think this answer is useful

for var in range(10,-1,-1) works

The Answer 9

1 people think this answer is useful

Short and sweet. This was my solution when doing codeAcademy course. Prints a string in rev order.

def reverse(text):
    string = ""
    for i in range(len(text)-1,-1,-1):
        string += text[i]
    return string    

The Answer 10

1 people think this answer is useful

You can always do increasing range and subtract from a variable in your case 100 - i where i in range( 0, 101 ).

for i in range( 0, 101 ):
    print 100 - i

The Answer 11

1 people think this answer is useful

I wanted to loop through a two lists backwards at the same time so I needed the negative index. This is my solution:

a= [1,3,4,5,2]
for i in range(-1, -len(a), -1):
    print(i, a[i])


-1 2
-2 5
-3 4
-4 3
-5 1

The Answer 12

0 people think this answer is useful

I tried this in one of the codeacademy exercises (reversing chars in a string without using reversed nor :: -1)

def reverse(text):
    chars= []
    l = len(text)
    last = l-1
    for i in range (l):

    result= ""   
    for c in chars:
        result += c
    return result
print reverse('hola')

The Answer 13

0 people think this answer is useful

Oh okay read the question wrong, I guess it’s about going backward in an array? if so, I have this:

array = ["ty", "rogers", "smith", "davis", "tony", "jack", "john", "jill", "harry", "tom", "jane", "hilary", "jackson", "andrew", "george", "rachel"]

counter = 0   

for loop in range(len(array)):
    if loop <= len(array):
        counter = -1
        reverseEngineering = loop + counter

The Answer 14

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You might want to use the reversed function in python. Before we jump in to the code we must remember that the range function always returns a list (or a tuple I don’t know) so range(5) will return [0, 1, 2, 3, 4]. The reversed function reverses a list or a tuple so reversed(range(5)) will be [4, 3, 2, 1, 0] so your solution might be:

for i in reversed(range(100)):

The Answer 15

-1 people think this answer is useful

You can also create a custom reverse mechanism in python. Which can be use anywhere for looping an iterable backwards

class Reverse:
    """Iterator for looping over a sequence backwards"""
    def __init__(self, seq):
        self.seq = seq
        self.index = len(seq)

    def __iter__(self):
        return self

    def __next__(self):
        if self.index == 0:
            raise StopIteration
        self.index -= 1
        return self.seq[self.index]

>>> d = [1,2,3,4,5]
>>> for i in Reverse(d):
...   print(i)

The Answer 16

-2 people think this answer is useful
a = 10
for i in sorted(range(a), reverse=True):
    print i

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